Concatenation

Segmenting long messages

Concatenation is a process by which long messages are broken up into segments and then stitched back together for the recipient. Concatenation is done automatically by carriers that support it. If a message can't be concatenated, then the receiving device will get the message in segments. Each segment includes a header noting the intended reading order of the message, for example 1/3, 2/3, 3/3. All FreeClimb phone numbers support concatenation for both inbound and outbound messages.

Carrier support

All major US carriers, except US Cellular, support concatenated SMS. Many smaller carriers also support concatenation. As of March 2019, Sprint supports concatenated SMS, although some older Sprint devices may still display long messages in segments.

Breaking down the process

A single SMS segment can contain 140 bytes. Messages with more than 140 bytes are automatically spread across multiple segments. Each segment in a multi-segment message has a 6-byte User Data Header (UDH) added to it. The carrier uses the UDH to reassemble the segments in the correct order for the recipient.

At 8 bits per byte, a single segment can have up to 1,120 bits. The default encoding scheme, GSM 7, uses 7 bits to encode a single character. Therefore, GSM encoded messages can have 160 characters in a single segment. On the other hand, Unicode uses 16 bits to encode a single character, which means each segment can only contain 70 characters. When the bytes taken up by UDH are factored in, the character count decreases to 153 characters per segment for GSM and 67 characters per segment for Unicode. For more details on GSM and Unicode encoding, please reference our encoding guide.

Outbound messages

While concatenation or dividing a message into segments are automatic, there are reasons to be aware of the number of segments your message will have. One reason is deliverability. The more segments there are to stitch back together, the more likely it is that the carrier misses a segment or displays it in the wrong order. A good rule of thumb is to keep your messages to 10 segments or fewer. Feel free to use this handy tool to check how your message will be split.

Inbound messages

Inbound messages over 160 characters long will be received on your FreeClimb number as one single message, as long as the sending carrier supports concatenation.